

Gluetun has an example for qbt port forwarding integration on the wiki. I’ve been using this for about a year and some without problem (to my knowledge at least). I’ve seen that gluetun will retry setting the port in qbt for a while to work around the timing issues.
They also mention a bug in qbt when changing the port, which is also covered in the example.
Edit: I see you’re already using the port down script. Not sure what else could be the root cause if you’re running on an up to date container version :/
I think mine was in Akihabara last year for some (dirt cheap) second hand Switch games, but besides that I have no idea where or when.